Friday, April 17, 2015

Dusty Snow: The Monster in the Basement

"A very strong cold front arrived on April 15th with near hurricane velocity winds. The storm knocked down trees, signs and power poles all over Utah and filled the air with choking dust. In the next week, 42 inches of snow fell on top of the chocolate brown layer of dust. Backcountry recreationists had their last fling with some of the best powder of the season on mostly stable snow. Then, finally, spring arrived."

Sound familiar? That quote is from the Utah Avalanche Center 2001-2002 annual report. The very strong cold front on April 15th was produced by the 2002 Tax Day Storm, which like the storm we experienced earlier this week, was a huge dust producer.

AVHRR satellite imagery at 2046 UTC 15 April 2002. Dusty areas not obscured by clouds identified with warm colors in inset at upper left. Source: West and Steenburgh (2010).

The snow that year was remarkably brown once we were well into the melt season. We are guaranteed a repeat of that this year, at least in those areas where a snowpack remained prior to this week's storm.

One of our readers, Scot Chipman, sent me the photo below of the snowpack at Snowbird (Volkl thanks Scot for his careful product placement!). 26 inches of beautiful white snow on top of a layer of dust deposited on the snowpack by wind and falling snow during Tuesday afternoon and evening.

As that white snow melts, it will percolate through the snowpack and eventually the dusty layer (a.k.a. snirt - part snow part dirt) will be sitting on top. Here's how things looked in May last season.

Yes, that was snirty, but this year will be far worse. I wonder how tolerable late-season tours are going to be once the snirty monster in the basement is sitting on top.

A few people have asked me if all this dust is due to the drought. Drought may be an exacerbating factor, but the primary factor in dust storms in Utah is strong flow over a disturbed land surface. Cryptobiotic crusts (pictured below), desert pavement, and other natural surfaces in the desert west are very resistant to erosion. However, as can be seen by the trails below, they are also easily disturbed.

Thus, we have had significant dust deposition even in relatively snowy/wet seasons. The photo below was taken at Snowbird in June 2011, at the end of a remarkably snowy/wet season. Plenty of dust.

Further, the processes influencing dust emissions from playas (i.e., dry lake beds) are very complex and not as straightforward as the surface being wet or dry (see Reynolds et al. 2007 for an example).

The good news is that the snirt is temporarily buried under the new snow in most areas. Instead of ski it if it's white, try ski it while it's white.

2 comments:

Another high-quality post, Jim. Thanks!! As a resident of the Salt Lake Valley, one of the big questions I have regarding dust on snow in the Wasatch is how much of it comes from the gravel quarries at Point of the Mountain in the south valley. Talk about a dust injection system located just upwind of the greatest snow on earth!!

On a larger scale, another thing that I think would be more informative to look at to understand the trend of dust generation (is it going to get worse?) would be to look at changes in land-cover over time (e.g., how much land has gone from undisturbed vegetative surface to roads, drill pads and urban development), and also drought-- how does soil drying impact dust generation? These would be very good GIS-based natural resource/climatology research questions for a graduate student, I'd think...

That quarry is a big dust producer for residents in and around point of the mountain, but it is probably not a significant contributor to the big storms we get from time to time, especially in the spring.

Your satellite-based effort would be a great undertaking. It's not the sort of thing that my group does, but maybe others on campus can get on it.

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The Wasatch Weather Weenies discuss the weather and climate of the Wasatch Front and Mountains, western United States, and beyond.

Participants include aspiring and old-school atmospheric scientists, weather enthusiasts, powder snobs, and poor souls enrolled in classes taught by University of Utah Atmospheric Sciences Professor Jim Steenburgh. Many posts feature content or insights enabled by the support of the National Science Foundation, Office of Naval Research, and the NOAA/National Weather Service.

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